My best one wasn't actually witnessed happening, but I wish it had been caught on camera, because I'd have loved to see it!
Out on a rural ride with some friends, most of the group were ahead, two chaps some way behind chatting, I decided to stop and take my jacket off. Pulled over to the side of the road, and put my foot down on the grass verge, which was higher than the tarmac. Except it wasn't, it was lower, and the grass was long. So I out a foot out, started to lean, carried on leaning, and some more, then my foot hit bottom and I went right over sideways, 180 degrees, and ended up upside down, in the ditch under the hedge, still 'on' my bike, feet on pedals, wheels spinning lazily.
Not only that, but I was wedged, and couldn't get up. Luckily the two chaps behind came along in a matter of seconds and hauled me out. I'd love to see it happen, because it's probably the most graceful and athletic thing I've ever done!
I usually look like I've been dragged through a hedge, that day, I had an excuse!